It has been known to use separate electrically connected filters where several pass bands are required. For example, in cellular radio telephone systems, there is a need to be able to discriminate signals in two relatively narrow pass bands. These pass bands are separated by an intervening stop band.
The use of two parallel, individual, single pass band filters to provide a dual pass band function suffers from a need to provide enough physical space for two separate filters. Further, there is a phasing requirement which must be met where two separate single pass band filters are coupled in parallel.
Another disadvantage of using two separate filters is that the pass band insertion losses will be greater than desired. Finally, two different sets of filter elements are required for two separate filters, one set resonant in one pass band, the other set resonant in the other pass band.
Thus, there continues to be a need for a poly-band filter structure which provides a multiple pass band function implemented in a single filter. Preferably, such a filter will be smaller physically than two individual single pass band filters which have been connected in parallel. Further, such a filter will preferably have a lower insertion loss and better selectivity than the two separate filter embodiment. Finally, such a filter might have only one set of filter elements, which all resonate at the same frequency.